Why does Strephon look like he is 25?

Question

Why does the council of peers correctly assume Strephon's age? Stephen is fairy down to his waist due to his fairy mother but below that is human. Also it is clearly stated that all fairies appear to be 17. Thus by logic Strephon should appear to be 17 (from the waist up anyway). However the peers seem fairly certain that Strephon is around 25. This is not really important to the plot as the idea of Iolanthe having the baby at age 0 would still be only slightly less preposterous than her being his mother as well as younger than him, but it is a possible oversight none the less.

Why does he not look like a 17 year old (from the waist up)

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Answer 1

He is 25. That's why he "looks" 25.

25-year-olds look very similar to 17-year-olds [citation needed], so even if, from the waist up, Strephon looks like he's 17, the people who are calling him 25 have known him (or at least known of him) since his birth, 25 years ago. Since they are relatively logical people (don't think about this too much), they assume that he is 25 years old, as he is. In addition, Strephon has probably told Phyllis his age before they got engaged (although engagements can be pretty quick in these operettas), and she has probably told the Lord Chancellor. Here's what the LC says about Strephon:

She’s his mother – and he’s nearly five-and-twenty!

He seems certain. Even if he's not, he knows that Phyllis wants to marry him. While spouses of 17 are great for 21-year-olds [citation: Pirates of Penzance], a 25-year-old would look for someone in her own age range.

Also, it's Gilbert and Sullivan, so you can't think about it too much. [citation: Pinafore, where Ralph and the Captain are the same age and also 20+ years apart...]

Or maybe they judge a man's age by his legs. Or they can tell a man's age in half a minute, and they do.